Use of nondestructive test methods to determine the thickness and compressive strength of unilaterally accessible concrete components of building

1 Introduction

Nondestructive test methods have been commonly used in the building industry for decades. They are used in the broadly understood diagnosis of built structures, being particularly useful in testing structural components made of concrete [1,2,3]. Considering the unique structure of the building investigated as part of this study, especially the fact that its concrete core was only unilaterally accessible and no boreholes could be made in it, the authors concluded that it would be most expedient to use the sclerometric method and the ultrasonic method to determine the thickness and compressive strength of the building’s core. The methods are described further in this article. As the investigated building is rare not only in Poland, but also in the world because of its atypical structure, the authors decided to describe it in more detail.

The Cable-Supported House, informally referred to as “Trzonolinowiec,” is a modernistic building located at 72–74 Tadeusz Kościuszko Street. It was built in the years 1961–1967 [4,5]. A photograph of this building is shown in Figure 1. The Trzonolinowiec has 12 storeys accommodating 44 flats. On the ground floor level there is a service storey, while above the roof level, within the building core outline, a lift machine room is situated. The building was erected according to a design by Andrzej Skorupa and Jacek Burzyński [4,5], which, however, had been considerably modified and impoverished by, among other things, reducing the amount of glazing [4,5]. It is one of the few buildings of this type in Europe. The building was awarded the title of the Wroclaw House of the Year for 1967 [4].

Figure 1

View of Trzonolinowiec in Wroclaw (photograph by T. Gorzelańczyk).

The building’s basic structural components are a reinforced concrete core and a system of 12 steel cables. The core is 42.0 m high, and its outer dimensions are 5.10 m × 5.10 m. It was founded on a 1.20 m thick 12.0 m × 12.0 m base slab. The building’s core includes a reinforced concrete lift shaft, a staircase, a rubbish chute shaft, and service shafts (in the N and S walls). The floor slabs of the storey landings are monolithic and made of reinforced concrete and constitute an integral part of the core, also as regards its bracing. The empty spaces between the floor slabs and the ceiling slabs are used as ducts for pumping hot air from the radiant ceiling-floor heating system used.

The steel cables are anchored to the core’s top edge, and their cross section varies from 127 steel strands ϕ5 mm on the topmost storey to 19 steel strands ϕ5 mm on the bottommost storey.

The prefabricated triangular load-bearing floor slabs are 4 cm thick and have the dimensions of 2.4 m × 4.3 m and 2.35 m × 4.2 m. They are strengthened with 19 cm × 4 cm ribs. The slabs were on-site monolithically integrated into four-piece squares and their one edge or corner on the one side was rested on the core, whereas their other corners were suspended using steel cables. Three-centimetre thick ceiling slabs (these slabs are not structural components, carrying only their own weight) with 7 cm × 4 cm ribs were suspended from the floor slabs.

The curtain walls are made of three-layer PW-8 panels, insulated with polyurethane and mounted on the load-bearing steel frame structure. Originally, they were externally clad with flat asbestos cement sheeting. The partitions were made of 12 cm-thick aerated concrete. In the case of the walls between dwelling units, they were made as double with an inner layer with a felt insert.

The building was erected using an innovative method combining monolithic reinforced concrete structures with the slipform technique and the lifting of floor slabs suspended on cables using hydraulic cylinders designed by T. Tenidowski [4]. First, the basement storey was built and a base slab for founding the core (a basic component of the load-carrying system of Trzonolinowiec) was prepared. The core was erected using slipform shuttering. After the core’s load-bearing walls were built within a given storey, the shutters were moved one storey up. Simultaneously, using hydraulic jacks, the erection head was raised, and together with it, the floor slabs (cast in situ on the ground floor level) of the successive storeys were pulled up on 12 steel cables. The latter were passed through holes in the slabs so that each of the cables had its one end under the bottommost slab while its other end was fixed to the head placed on the topmost slab. The successive “layers” of slabs were supported on the successively cast levels of the core. After all the dwelling storeys had been built, the core’s bottommost storey (the ground floor storey) was cast. Once the building had been erected the cables (cable stays) were tightened and anchored in the foundation and in the head. Then, the floor slabs were joined to the core and the cables were protected with an asbestos concrete surround [6]. Archival construction progress photographs showing the stages in the erection of the building are presented in Figure 2. The authors obtained the photographs from the collections of the Museum of Architecture in Wrocław.

Figure 2

Archival construction progress photographs showing stages in Trzonolinowiec erection (Collection of the Museum of Architecture in Wroclaw).

In the years 1975–76, the building underwent a general overhaul whose aim was to brace the structure as the building had been found to vibrate excessively under wind loading. The vertical sections of the load-bearing cables were encased in reinforced concrete columns (25 cm × 25 cm in cross section) in order to rest the slabs on the columns so that they were no longer suspended on the cables. In the ground floor and basement parts, the cables were encased with C240 steel sections.

After several decades of being in service and neglect due to no current and maintenance repairs, the building reached an imminent failure condition, which means that at any moment, a failure of the structural components (a structural collapse) endangering the life and health of the residents can occur [7].

Because of the above, the city authorities and the building manager took action to assess the condition of the building and its repair. For this purpose, a research team made up of scientists from the Civil Engineering Faculty of Wroclaw University of Science and Technology, headed by Prof. Krzysztof Schabowicz, was appointed. The authors carried out a considerable number of tests in the building, including nondestructive tests. One of the elements of the condition survey report being prepared was to make a model of the building using Building Information Modelling (BIM) [8,9]. This model was then to be used to prepare a model for a numerical FEM analysis and structural calculations of selected structural components. In order to make a BIM model information about, among other things, the thickness of the building’s concrete core and its compressive strength was needed. For this purpose, nondestructive test methods, i.e. the sclerometric method and the ultrasonic method, were used. One should note that the building’s core was unilaterally accessible (only from the staircase), which made it difficult to estimate the thickness of the core’s walls. Moreover, no concrete core samples could be taken from it.

The literature shows that non-destructive methods are widely used in the diagnosis of concrete elements, including the localization of various defects (especially cracks) and inclusions [10,11,12]. They can also be used successfully to assess the quality of repairs to concrete elements, as shown, for example, in the work [13]. Non-destructive methods are commonly used in the testing of road infrastructure elements [14] and concrete with various fibres [15]. The NDT methods are highly suitable for estimating the thickness of unilaterally accessible walls [16]. The latest acoustic NDT methods, such as ultrasonic tomography, which is currently increasingly commonly used for testing the condition of structures made of concrete [17,18], including massive members of hydroelectric power plants accessible only unilaterally [19], can be useful for this purpose. The usefulness of the ultrasonic tomography method was proven by Gorzelańczyk et al. [20,21], who successfully determined the thickness of the concrete walls of a heat pipe-carrying tunnel and that of the concrete shell of a mine lift shaft. To determine the compressive strength of concrete in structural components, it is recommended to use the sclerometric method and the ultrasonic method. The methods have been known and successfully used for several decades [1,15]. They are especially recommended in a situation when no core samples can be taken from the structure and tested in a testing machine to determine the compressive strength [22,23]. It should be noted here that further in this article, only tests carried out using the sclerometric method to determine the strength of the concrete are presented. Despite the many attempts made using an ultrasonic concrete tester and the surface transmission method, the strength of the concrete could not be reliably estimated. Due to the unilateral access to the tested concrete members of the building’s core, it was also impossible to conduct tests using the direct transmission method by placing ultrasonic transducers opposite each other on the two sides of the tested member.

At the end of this section, it is worth emphasizing that the authors did not aim to indicate a gap in knowledge because the non-destructive methods used in the work are widely known in the literature. It was important for the authors to demonstrate that the selected methods will be successfully applied to the object in question, especially due to the one-sided access to the tested elements.

2 Description of tests

The nondestructive equipment, an Acoustic Control System Mira A1040 ultrasonic tomograph and a Schmidt hammer type N, shown in Figure 3, was used to test the building’s concrete core.

Figure 3

Nondestructive equipment used for testing: (a) ultrasonic tomograph and (b) type N Schmidt hammer.

The ultrasonic tomograph is designed for testing concrete, reinforced concrete, and stone structures with unilateral access to the tested object in order to evaluate the integrity of its material, detect foreign inclusions, holes, defects, cracks, honeycombs and bars within the material and measure the thickness of objects. The tomograph can test objects up to 2 m thick. The test results are displayed as a cross section (image) type B, which makes it easier to interpret and analyse them. Specialist software makes it possible to reproduce an image from a data file and create a 3D picture of the inner structure. The tomograph has an integrated head, i.e. an antenna array, which includes 48 elements (12 blocks with 4 elements in each block). These are broadband DPC transducers (needing no coupler) of low-frequency transverse waves with a non-abrasive ceramic pad. Each transducer has an independent spring suspension whereby it can be pressed to the tested surface. The head’s nominal operating frequency is 50 kHz [24,25].

Nondestructive tests by means of the ultrasonic tomograph were carried out on the core’s concrete walls by the staircase in 196 measuring places. The tests were conducted in four bands (four measuring points in each band) on the core’s wall by the stairs and in one measuring point on the wall at each flat entrance door. The measurement points on each band were approximately 50 cm apart from each other, counting from the bottom. The lowest point was approximately 20–30 cm above the ceiling level. The authors wanted to obtain images of almost the entire height of the core wall. Exemplary bands with measuring points are shown in Figure 4. The device’s testing depth was set at 750 mm.

Figure 4

Location and general denotation of measuring points in the staircase (projection drawing is from the study of Schabowicz et al. [7]).

Nondestructive tests by means of the N-type Schmidt hammer were carried out in order to determine the compressive strength of the concrete. The medium (normal) n-type hammer with an impact load of 2.25 Nm is designed for testing ordinary concrete in precast concrete units and in structures. Schmidt hammers determine the surface hardness of concrete by measuring the rebound of the hammer impact mass. The rebound value, i.e. so-called rebound number R is read off the hammer’s scale. In order to determine the compressive strength of concrete by the sclerometric method using the Schmidt hammer, one must use an empirical f _c – R relation (between concrete’s compressive strength f _c and rebound number R) [22,23]. This relation was determined using the approximate method by matching an empirical basic regression curve to the composition, the concrete making process and the curing, age and moisture content of the concrete. This method has been used in Poland since about 1960, i.e. from the very beginning of the application of NDT methods to concrete strength evaluation [23]. As mentioned earlier, it was not possible to take core drillings from the concrete shaft. Therefore, the empirical relationship was taken from the curves developed by ITB contained in the study of Runkiewicz and Sieczkowski [23] for concrete of similar composition, made at a time similar to that of the construction of the tested object. The tests using the Schmidt hammer were carried out in the selected places marked in Figure 4.

3 Processing of sclerometer measurement results

The measurement results were recorded on special forms (sclerometric measurement logs), appended to this article. The preparation of output data includes calculations of the medians of the readings from the particular testing places. As a result, one obtains n values of R _i .

Having n values of R _i for a given element (area), the following statistical parameters were calculated [23]:

• the median of rebound numbers R _m ,

• the standard deviation (s _R ) of the medians of the rebound numbers

where the coefficient of variation (ν _R ) of the medians of the rebound numbers is calculated from the relation

where R _i is the median of the rebound numbers in the measuring place, R _m is the median of the medians of rebound number values in n measuring places, n is the number of measuring places, s _R is the standard deviation of rebound numbers R _i in n measuring places (points), and ν _R is the coefficient of variation of rebound numbers R _i in n measuring places (points).

Then, using the parabolic relation f–R mean strength f _m (for cylindrical samples) was calculated from the following formula dedicated to type N sclerometers [22]:

When calculating the ultimate value of f _m , the correction coefficients given in the study of Runkiewicz and Brunarski [22] were applied to take into account the age of the concrete and the degree of moisture accumulation in it.

Then, indicators characterizing concrete quality, such as

• strength standard deviation s _f ,

• strength variation coefficient ν _f , and

• concrete homogeneity

When parabolic relations f–R were used, strength standard deviation s _f was calculated (for a type N sclerometer) from the following formula [23]:

The coefficient of variation of concrete strength was calculated using the following formula:

The homogeneity of the concrete was determined on the basis of variation coefficient ν _f , using Table 1 [23].

The final step in the processing of the results was the evaluation of the characteristic compressive strength of the concrete in the structure. The characteristic compressive strength of the concrete in the structure (f _ck,cyl) according to the design standards was determined on the basis of the sclerometric tests as the smaller relation from the following [23]:

where f _ck,is,R is the characteristic compressive strength of the concrete in the structure, determined on the basis of the sclerometric tests, meeting the design standards; and f _m(n),is,R is the mean compressive strength of the concrete in the structure, determined indirectly on the basis of the sclerometric tests.

Knowing the value of f _ck,cyl, the strength class of the concrete was determined in accordance with standard PN-EN 206 [26,27].

4 Test results and their analysis

4.1 Tests using ultrasonic tomography

The nondestructive test results (in the form of images) obtained using ultrasonic tomography in all the 196 measuring points are reported in the annex to the study of Schabowicz et al. [7]. For reasons of space, the test results together with their interpretation are presented below for only a few selected measuring points.

Figures 5–9 show type B images for selected measuring points located on the core wall in the staircase. Figure 10 shows the image for the measuring point located on the side wall at the entrance to a flat on the seventh floor. In the figures, the determined thickness of the concrete wall is indicated by dashed lines and arrows.

Figure 5

Image obtained from tests using ultrasonic tomography in measuring point X2 (between ground and first floors) with indicated determined wall thickness.

Figure 6

Image obtained from tests using ultrasonic tomography in measuring point X4 (between first and second floors) with indicated determined wall thickness.

Figure 7

Image obtained from tests using ultrasonic tomography in measuring point X2 (between second and third floors) with indicated determined wall thickness.

Figure 8

Image obtained from tests using ultrasonic tomography in measuring point X13 (between seventh and eighth floors) with indicated determined wall thickness.

Figure 9

Image obtained from tests using ultrasonic tomography in measuring point X14 (between ninth and tenth floors) with indicated determined wall thickness.

Figure 10

Image obtained from tests using ultrasonic tomography in measuring point X17 on the side wall at entrance to flat on the seventh floor, with indicated determined wall thickness.

An analysis of the obtained images showed that the thickness of the core’s concrete wall varies along the height of the building. The largest core wall thickness, amounting to 350–400 mm, was found at the height between the ground floor and the first floor. At the height between the first floor and the second floor wall thickness amounts to 280–300 m. In the case of the next floors, wall thickness ranges from 140 to 200 mm. It should be noted that the core wall thickness markedly decreases with the building’s height, reaching 140–160 mm. A detailed breakdown of the thickness of the core’s concrete walls is presented in Table 2.

Table 2

Breakdown of core concrete wall thickness determined using ultrasonic tomography

Measuring place	Determined wall thickness (mm)
Core wall between ground floor and first floor	350–400
Core wall between first and second floors	280–300
Core wall between second and third floors	180–200
Core wall between third and fourth floors	150–180
Core wall between fourth and fifth floors	150–180
Core wall between fifth and sixth floors	150–180
Core wall between sixth and seventh floors	150–180
Core wall between seventh and eighth floors	150–180
Core wall between eighth and ninth floors	150–180
Core wall between ninth and tenth floors	140–160
Core wall between tenth and eleventh floors	140–160

In the case of the “side” walls in the core, in which there are entrance doors to flats, their thickness was found to range from 140 to 160 mm. Moreover, two control boreholes were drilled through the core’s walls from the staircase, confirming the wall thickness values estimated using ultrasonic tomography.

4.2 Sclerometric test

The sclerometric test results for each of the measuring points are reported in the study of Schabowicz et al. [7]. They are contained in sclerometric measurement logs. For each of the measuring points, the value of f _ck,cy was determined, and then the concrete’s strength class in accordance with standard PN-EN 206 [26]. An exemplary measurement log for tests carried out on the core’s concrete wall between the fifth and sixth floors is shown in Figure 11.

Figure 11

Sclerometric measurement log for measuring points located on the core’s concrete wall between the fifth and sixth floors.

An analysis of the results of the sclerometric tests carried out on the core’s concrete walls in 10–20 measuring points along the building’s height from the ground floor to the 11th floor clearly showed a slight variation in the concrete’s compressive strength. For most of the measuring points, the concrete’s strength class determined according to PN-EN 206 [26] is C12/15. In a few places, the test results indicated concrete strength class C16/20 (between fourth and fifth floors, between sixth and seventh floors and between eighth and ninth floors). Namely, small differences in the specific strength of concrete most likely result from the fact that the concrete core was made in a dozen or so stages (described in Section 1). Considering the realities in Poland at that time, it probably occurred that the regimes related to the production and control of the concrete mix were not maintained, hence these differences. However, since in most of the measuring places the concrete strength class was established to be C12/15, it was the latter which was assumed in the structural calculations in the study of Schabowicz et al. [7]. Moreover, one should note that the assumption of an “underestimated” concrete strength provides a larger safety margin during structural calculations, which is particularly important in the case of old structures whose engineering documentation is not available and there is no certainty that their concrete members had been properly made and are characterized by the strength parameters specified in the design. It should be noted that sclerometric tests were conducted not only on the walls of the building’s concrete core but also the core on basement level, the walls under the lift shaft and the concrete columns created by concreting the load-carrying cables as part of the repair in the 1970s were tested.

5 Conclusion

The results of tests carried out on the core of the Trzonolinowiec building located in Wroclaw have been presented. The nondestructive ultrasonic tomography method and the sclerometric method were used for the tests.

The thickness of the unilaterally accessible core of the building was determined using ultrasonic tomography method. An analysis of the images showed that the thickness of the core’s concrete wall varies along the height of the building. The largest core wall thickness, amounting to 350–400 mm, was detected at the height between the ground floor and the first floor. At the height between the first floor and the second floor, the wall thickness amounts to 280–300 m. For the next floors, the wall thickness ranges from 140 to 200 mm. It should be noted that the core wall thickness markedly decreases with the building’s height, reaching 140–160 mm. The tests have clearly showed that the ultrasonic tomography method is highly useful for determining the thickness of unilaterally accessible concrete members.

The tests using the sclerometric method carried out on the concrete walls of the building’s core clearly showed a slight variation in the concrete’s compressive strength. In most of the measuring points, the concrete was found to be of class C12/15. In a few places, the concrete was found to be of class C16/20.

The results obtained by nondestructive testing were then used to build a BIM model of the Trzonolinowiec building. An exemplary visualization and cross sections made in the study of Schabowicz et al. [7] are shown in Figure 12.

Figure 12

BIM model: (a) horizontal cross section through building’s typical floor and core on level 3, (b) view from SW, and (c) vertical EW cross section through building [7].

One should note that the tests presented in this article were only a part of the tests carried out in the investigated building. All the tests and their results are presented in an extensive building survey report [7]. The test results presented in this study were used in structural calculations. The results of the calculations are very interesting and of critical importance for the further use of this building. They have clearly shown that the building is in imminent failure condition and immediately needs a major overhaul. The tests have provided extensive material for further publications by the researchers taking part in the investigations of the Trzonolinowiec building. In conclusion, the authors would also like to mention that the conducted research may also suggest areas for further research. Namely, the methods used in the study can be successfully applied to the diagnosis of historic buildings. They are particularly recommended when it is not possible to take samples to determine the thickness of the tested elements and to estimate their strength using destructive methods.